Fix: Use correct oldest result date in vulns

When parsing the result dates for vulnerabilities, the correct field is
now used for the oldest result.

When date parsing was added to the Vulnerability model class, the newest
date was used accidentally.

src/gmp/models/vulnerability.js

@@ -27,7 +27,7 @@ class Vulnerability extends Model {
 
     if (isDefined(ret.results)) {
       ret.results.newest = parseDate(ret.results.newest);
-      ret.results.oldest = parseDate(ret.results.newest);
+      ret.results.oldest = parseDate(ret.results.oldest);
     }
 
     return ret;